This invention relates to an improved cylinder lock construction having a plug which is removable by means of a proper pass key.
It has long been recognized that a cylinder lock having a removable plug is a desirable product. For example, one may remove the plug in order to substitute another plug having a different key bitting arrangement. Alternatively, the plug may need to be removed for repair or replacement. Various prior art patents teach such removable plug constructions including U.S. Pat. No. 4,398,405 issued Aug. 16, 1983 for a "Lock with Key Controlled Removable and Installable Plug and Key for Same" as well as U.S. Pat. No. 4,416,129 issued Nov. 22, 1983 for a "Cylinder Lock with Key Removable Plug". The text of the two patents referenced is incorporated by reference. The patents are believed typical of the prior art relating to the subject matter of the present invention.
With respect to both of the referenced patents, the removable plug includes a series of combinating tumblers in the form of disc tumblers which are spring biased into engagement with the cylinder to thereby lock the plug to the cylinder. The combinating tumblers may be translated to an unlocked position, permitting plug rotation relative to the cylinder, by insertion of a proper key that engages the combinating tumblers and moves them against the spring biasing force. The removable plug also includes, typically, a control tumbler which is responsive to a special control key. The control tumbler is normally spring biased into an annular groove on the inside surface of the cylinder to prevent removal of the plug from the cylinder. The plug may be rotated but not removed. The control tumbler can be released from that groove or slot only when engaged and moved out of the groove by a particular control key. Insertion of the control key thus will move the control tumbler against a spring biasing force acting on that tumbler and effect removal of the tumbler from the groove permitting consequent release of the plug from the associated cylinder.
The prior art teachings perform in a satisfactory manner. However, in the references identified, biasing means such as a spring is utilized to engage the control tumbler. In the event the spring breaks, the plug associated with the cylinder may be accidentally released. Additionally, the use of a spring in association with a control tumbler constitutes an extra part necessary for the operation of the lock.
Against this background, the removable plug lock of the present invention was developed. With the lock of the present invention, a spring associated with the control tumbler has been eliminated thereby eliminating a part, but not the function associated with that part. The present invention also provides additional benefits with respect to the security and operability of the plug removable lock assembly.